Glass fiber reinforced thermoplastic cellular plastics

ABSTRACT

Glass fibers are coated with uncured thermosetting resin selected from the group consisting of glycerol triacrylate, ureaformaldehyde and phenol-formaldehyde of the type designated as B stage. About 5 percent by weight of the coated fibers, while cool, are introduced into a mixture of gas and molten thermoplastic material selected from the group consisting of polypropylene nylon and polystyene shortly (i.e. less than a minute) prior to entry into the mold of low pressure injection molding apparatus. The heat of the molten thermoplastic material brings about a curing of the thermosetting coating on the glass fibers, but such curing is so slow that it becomes effective at about the same time as the thermoplastic material cools and solidifies in the mold. The product, a thermoplastic structural foam having short fibers of thermoset-coated glass fibers distributed throughout, has greater stiffness and greater tensile strength than conventional structural foam.

States Patent 11 1 1111 Greeulber Ma 22 1973 [54] GLASS FIBER REINFORCED3,378,612 4/1968 Dietz ..264/328 x THERMOPLASTIC CELLULAR 3,561,0472/1971 Mclnnis 264/329 X PLASTICS P E Do ald J A ld rlmary xammer n mo[76] Inventor. Walter H. Greenberg, 220 Miller Assistant Examiner paulLeipold Road, Syossett, NY. 11801 Atmmey John R Ewbank [22] Filed: Apr.13, 1970 [21] Appl. No.: 28,087 I ABS RAC Glass fibers are coated withuncured thermosetting resin selected from the group consisting ofglycerol [52] f ggfig i triacrylate, urea-formaldehyde andphenol-formal- 51 I t Cl [D B29! 27'/00 dehyde of the type designated asB stage. About 5 perd 328 45 cent by weight of the coated fibers, whilecool, are in- 1e ZZZ/G 35 329 DIG f troduced into a mixture of gas andmolten ther- 260/2 5 3 moplastic material selected from the groupconsisting of polypropylene nylon and polystyene shortly (i.e

. less than a minute) prior to entry into the mold of low [56]References cued pressure injection molding apparatus. The heat of theITE STATES PATENTS molten thermoplastic material brings about a curingof the thermosetting coating on the glass fibers, but such 3,l64,563l/1965 Maxwell ..264/l43 X curing is so Slow that it becomes effectiveat about the g t same time as the thermoplastic material cools and3382302 5/1968 III solidifies in the mold. The product, a thermoplastic3I0s41973 4/1963 Beckley 264/45 x Structural foam f' g fibers ofthermos- 2,s55,021 /1958 Hoppe ..264/ x coated glas fibers distributedthroughout, has greater 3,339,240 9/1967 Corbett ..l8/l3 P X stiffnessand greater tensile strength than conventional 3,531,553 9/1970 Bodkins..264/328 X structural foam.

3,436,446 4/1969 Angell .....264/328 X 2,979,431 4/l96 l Perrault ..264/X 1 Claim, N0 Drawings GLASS FIBER REINFORCED THERMOPLASTIC CELLULARPLASTICS BACKGROUND OF INVENTION Heretofore various procedures have beenproposed for preparing articles by casting or injecting moltencompositions into molds for cooling into solid articles. Conventionalplastic materials are injected under pressure such as 10,000 pounds persquare inch, thus requiring large clamping pressures for the mold. Lowerclamping pressures are achieved by lowering the viscosity of the moltenplastic composition by distributing a large number of tiny gas cellstherein. Structural foam is a term applied to products resulting fromlow pressure injection molding. The mold is filled with the mixture ofmolten plastic and gas within a brief period, usually less than aminute, and is cooled rapidly to provide an article sufficiently solidto be handled. The combination of the thermoplastic material and the gascells has generally provided an article having significant resiliencyand relatively low tensile strength. In production of some small solidthermoplastic articles, it has been feasible to employ pellets of amixture of resin and glass fibers. However, in apparatus in which gas isdispersed into molten plastic in an extruder, the glass fibers causetroublesome abrasion of the extruder and the fibers are significantlypulverized in the extruder.

DESCRIPTION OF PREFERRED EMBODIMENTS Thermoset plastics generally havecrosslinking potentialities for growth in at least three dimensions, asdistinguished from the predominantly two dimensional growth of polymersfor thermoplastic materials. The curing of the thermosetting resininvolves sufficient growth of the polymer in three dimensions that thestructure is not thereafter readily plasticized by heat. Some of thethermally induced reactions effective for curing the thermoset resinsare also effective in chemically bonding to another material, such as athermoplastic resin.

Glass fibers have been employed predominantly in the reinforcement .ofthermosetting resins such as epoxy resins. Chopped fibers providingshort staples of glass fiber have been marketed for use in reinforcingplastics. Certain such glass fiber staples have been provided with thincoatings of plastic, so that in making epoxy products, epoxy-coatedfibers could be employed to achieve more perfect bonding, and so thatphenol-formaldehyde coated fibers could be employed when bonding tophenol-formaldehyde was desired. Glass fibers have been marketed havingthermoplastic coatings for use in making thermoplastic articles.

In accordance with the invention, a thin coating of a B stage resin isapplied to glass fibers, and short lengths (i.e. staples) of the coatedglass fibers are dispersed as relatively cool fibers into a pre-formedmixture of gas and molten thermoplastic at a time less than one minuteprior to the completion of the mold-filling operation. The cool fibersare not instantly cured. The tem perature of the molten plastic issufficient to initiate the curing of the B stage resin coated upon theglass fibers. The B stage resin is cured to a greater extent during thehardening and cooling of the plastic article, and in curing bonds moresecurely to the thermoplastic structural foam surrounding each fiber.

EXAMPLE 1 Polypropylene is plasticized and melted to provide a lowviscosity stream, into which nitrogen is intimately mixed to provide astream having a lower viscosity by reason of the large number of smallgas cells uniformly distributed in the stream. The mixture flows into anaccumulating zone adapted to determine the shot size for the lowpressure injection step and mechanical mixers agitate the contents ofthe accumulating zone. After the desired amount of mixture of moltenpolypropylene and nitrogen has been pumped into the accumulating zone, acontrolled quantity of glass fiber staples coated with a B stagethermosetting resin derived from glycerol triacrylate is injected intothe molten plastic and distributed therethrough by the mechanicalmixers. The weight of glass fibers (excluding the coating) is about 5percent of the weight of the polypropylene batch. Within about 30seconds after the introduction of the glass fibers, the mixture isreleased for flow into the mold, so that the mold is filled with themixture within less than a minute after the introduction of the glassfibers. The temperature of the molten polypropylene is sufficient toinitiate the curing of the B stage resin, but the polypropylene startscooling in the mold before the completion of the curing of the B stageresin. Further cooling leads to the production of an article having theglass fibers distributed throughout the structural foam. The article isa reel upon which cord is wound, and is a structural foam replacementfor a wooden reel. The tensile strength of the article is greater thanan unreinforced product. The reel has significantly greater stiffness,and it is significantly more difficult to temporarily deform the surfaceof the article than to deform an unreinforced article or an articlecontaining the same amount of glass fibers having a thermoplasticpolypropylene coating. By a series of tests, it is established that thedegree of stiffenning of the article is about 50 percent greater for theB stage resin coated fibers, so that about 7% percent polypropylenecoated fibers are approximately equivalent to about 5 percent of the Bstage resin coated fibers. The article is removed from the mold and isfound to be effective as a reel upon which cord can be wound. By reasonof having a greater stiffness than the corresponding unreinforcedarticle, it is more competitive with wooden reels. The thickness ofsections necessary for achieving a desired stiffness can be engineeredto be less when using the glass reinforcement method of the presentinvention than when using unreinforced compositions. The reel has acombination of properties making it attractive for use wherever lightweight reels are suitable.

EXAMPLE 2 A large diameter gear for the transmission of hand power atlow speeds is made from structural plastic (moderate density foam) usingunreinforced nylon in the low pressure injection molding. Although saidnylon foam gear is suitable for some situations, it lacks the ruggednessand light weight desired for some gears. A product having about 7%percent glass fibers, each glass fiber being coated with nylon misciblewith the structural foam, provides a gear having acceptable properties.Glass fibers coated with B stage ureaformaldehyde resin are shown to beparticularly effective, permitting the use of only about 5 percent byweight of the nylon for achieving the stiffness and ruggedness desired.Nylon pellets are melted and the molten nylon is admixed withsuperatmospheric carbon dioxide to provide a pressurized compositionwhich at atmospheric pressure would constitute about 30 percent byvolume nylon and about 70 percent by volume carbon dioxide. A stream ofthe compressed mixture is advanced toward an accumulating zone andthrough a mixing zone, in which relatively cool glass fibers areinjected and dispersed by the action of mechanical mixing blades, thefiow rates providing about 5 percent by weight of glass fiber per 95percent by weight of nylon. Particular attention is directed to the factthat the glass fiber staples are coated with a B stage resin derivedfrom ureaformaldehyde. The elevated temperature of the molten nyloninitiates the curing of the B stage resin. The time required for thefilling of the accumulating zone and the subsequent filling of the moldwith the shot necessary for molding the gear is less than one minute,whereby the finalization of the curing of the coating is after thefilling of the mold, and while the nylon is cooling towardsolidification into a structural foam gear.

EXAMPLE 3 Polystyrene is melted under pressure and admixed withcompressed trifluorochloroethane. The mixture flows into theaccumulating zone of a cylinder in which the incoming mixture displacesa piston until the controlled shot size has been accumulated. Analternative accumulator chamber receives the flow of the mixture whilethe principal accumulator is not receptive. Meanwhile the articleprepared by a previous molding cycle is being removed from its mold, anda mold is being readied for injection. After the mold is ready toreceive a shot, the mixture of polystyrene and trifluorochloromethane isreleased from the accumulating zone and pumped by the action of thepiston toward the mold. Between the accumulating zone and the mold is amixing zone through which the stream has turbulent flow, and in thismixing zone, a quantity of glass fibers is injected to provideproportions corresponding essentially to approximately 5 percent glassfibers and 95 percent polystyrene. Particular attention is directed tothe fact that the glass fibers are coated with a B stage resin derivedfrom phenol-formaldehyde. The heat of the polystyrene initiates thecuring of the B stage resin, and tends to promote bonding reactions withthe polystyrene. The mold shapes the composition into semicylindricalpipe insulations members having greater stiffness than unreinforcedmembers.

Various modifications of the invention are possible, and the examplesare merely illustrative of some of the ways in which B stage coatedglass fibers can be dispersed in a molten plastic foam and cured byreason of the heat transferred to the fibers from the hot plastic foam,the time between the introduction of the fibers and the completion ofthe filling of the mold being less than 1 minute. I

The invention claimed is:

1. In the method in which a mixture of molten thermoplastic material andgas fills a mold by low pressure injection molding, the improvementwhich consist of:

preparing a mixture consisting essentially of molten thermoplasticselected from the group consisting of polypropylene, nylon, andpolystyrene, and gas selected from the group consisting of nitrogen,carbon dioxide, trifluorochloromethane, and mixtures thereof;

admixing, based upon the weight of thermoplastic,

about 5 percent by weight of short staples of glass fibers having acoating of B-stage thermosetting resin in a relatively cool conditionwith said mixture of molten thermoplastic material and gas, the heatfrom the thermoplastic material both initiating and bringing about thecompletion of the reactions for curing the thermoset coating on saidglass fibers, said B-stage thermosetting resin being selected from thegroup consisting of glycerol triacrylate, urea-formaldehyde, andphenolformaldehyde; and

transferring the mixture of glass fibers, thermoplastic material, andgas to said mold so that said mold is filled with gas-expandedcomposition within less than one minute subsequent to the admixing ofthe glass fibers with the thermoplastic material, said B-stage resinbeing cured and bonded to the thermoplastic material by the heat derivedfrom the molten thermoplastic during the time period ending by the timethe thermoplastic material has cooled to provide a cellular moldedarticle at ambient temperature, said molded article having a stiffnessand ruggedness approximately matching that of a molded articlecontaining about 7% percent glass fibers coated with the thermoplastic,such decrease in the amount of glass fibers being attributable to thethermoset coating on the glass fibers.

